The present invention relates to circuit fault detection, and more particularly to ground fault detection and location in normally ungrounded DC power distribution systems. Typically such systems have significant capacitive reactance components and are under the influence of strong electromagnetic fields. These situations are normally associated with utility power generation and distribution, industrial plants, and computer/electronic systems therein. In such systems, ground faults must be located without taking unaffected equipment out of service.
Generating stations and substations use 110 to 240 volt DC ungrounded battery systems to operate control systems and other DC devices. Some of the control systems are critical to plant operational integrity and must operate at all times. If two ground faults on opposite polarities of the same battery system occur simultaneously in the system, the battery may be shorted through ground. If two or more simultaneous ground faults on the same conductor occur, an undesirable bypassing of controlling devices may occur and cause malfunction or misoperation. Consequently isolation and repair of the first fault must, therefore, be performed as quickly and efficiently as possible to minimize the chances that the whole battery system will be shorted or become inoperative.
The major components of an ungrounded DC distribution system usually include the DC battery assembly and battery charger. Main source conductors connect the battery assembly to the circuit breaker of a multifeeder distribution panel, and the individual loads to those feeders. The type of loads associated with this system are motors, solenoids, relays, electronic monitoring equipment, and electronic control devices. A common characteristic associated with this type of system is, firstly, stray capacitance created by the distribution lines with respect to ground and, secondly, input capacitive reactance of the loads. The value of the stray capacitance ranges from a few picofarads to 200 microfarads or more. This is an important characteristic since it plays an important part in the type of test equipment that can be used to locate ground fault currents.
A basic problem in such systems is the need to identify low level DC fault currents, namely, low to high impedance ground fault currents in the presence of much larger DC load currents and electromagnetically induced noise currents.
U.S. Pat. No. 4,837,519 Lopetrone et al., assigned to the assignee of this application and incorporated herein by this reference, discloses ground fault detection using an impedance element connected across a DC power supply, an interrupter periodically connecting a tapping point of the impedance element to ground for producing a fault current signal when a downstream fault condition is present. A magnetic detector that operates synchronously with the interrupter senses the fault current signal when the detector is located between the impedance element and the fault condition. The magnetic detector is connected in a closed-loop, high gain analog circuit, being typically located for enclosing the positive and negative conductors of the system whereby the gross load current is cancelled. The circuit can then measure the fault current and provide indication of the fault. The '519 patent also discloses a control current device feeding an offset coil winding of the magnetic detector for adjustably biasing the detector against the effects of the gross load current, whereby the detector, which may be a portable unit, can be applied to only one of the positive and negative conductors.
The above-disclosed system as operationally implemented has an analog gain in excess of 1,000,000, being interfaced through an 8-bit analog to digital converter, the detector circuitry having sufficient dynamic range for directly measuring fault currents only when the load current is cancelled (the sensor enclosing positive and negative system conductors) or when the load current is balanced by the adjustable bias control current. This limited dynamic range reflects the relatively large load currents that can approach 100 amps which can be present, and the need to accurately measure fault currents as low as .+-.3 milliamperes. Thus the load current must be balanced to within approximately .+-.1 ampere for the system to work properly, a relatively difficult task when one line only is being sensed. The closed-loop, high gain circuitry is difficult to stabilize and calibrate, especially on account of phase shifts and heating associated with the offset coil winding.
The above-disclosed system, while providing useful ground fault detection and location in typical environments, still exhibits one or more of the following disadvantages:
1. It is expensive to produce and maintain in that the construction of the detector is complicated by the required coil for the control current, and because a high degree of skill is needed for setting up the high-gain closed-loop circuitry; PA1 2. It is difficult to use and/or ineffective in many situations wherein only one of the system conductors is available for sensing; and PA1 3. The sensitivity is marginal in some applications.
It is also known to utilize a charge transfer voltage controlled oscillator for measuring an unknown capacitance. It is believed that such techniques have not been extended by others to the problem of ground fault detection in DC power systems.
Thus there is a need for a portable ground fault detector that avoids the disadvantages of the prior art.